Hybrid Quantum Systems of Nitrogen-vacancy Centers in Diamond Coupled to Magnetic Insulator Materials

Nitrogen-vacancy (NV) centers in diamond have been known as excellent solid-state qubit platforms in quantum information science. Hybrid quantum systems of NV centers and magnetic insulator materials are of particular interest in recent years due to their great potential in computing and sensing applications. In this dissertation, we explore several experimental and theoretical aspects of the hybrid (or composite) quantum system of NV centers and yttrium iron garnet (YIG). YIG is a ferrimagnetic insulator material extensively studied in the field of spintronics and magnonics. It shows exceptionally low magnetic damping that leads to nontrivial magnon related phenomena such as the spin Seebeck effect. The first section of this dissertation focuses on the fundamentals of quantum physics. Chapter 1 provides an introduction to quantum information science and engineering. Chapter 2 describes the basics of NV centers. Chapter 3 features the concept of boson-mediated interaction of qubits. In Chapter 4, we show that the NV center can serve as a nanoscale temperature probe of the YIG substrate and is potentially useful for the study of the spin Seebeck effect. Employing an all-optical ratiometric thermometry technique, we find that the NV-based temperature sensing can function down to liquid nitrogen temperatures without a deterioration of its temperature sensitivity, which is ideal for the sensing application of YIG. With an array of NV centers embedded into a polymer membrane, we map out a temperature gradient of YIG as a demonstration of this all-optical temperature sensing. Chapter 5 pursues a potential of the hybrid quantum architecture of NV centers and magnons in YIG, with the expectation that long-distance two-qubits gates of NV centers will be enabled. We perform a theoretical study of a practical hybrid quantum system of NV centers interacting with magnons in YIG waveguide and bar structures. With both a semi-analytic analysis and a numerical analysis, we construct a framework to compute the NV-magnon coupling strength and the NV-NV effective coupling strength mediated by magnons. This will guide future experiments and device fabrications. Lastly in Chapter 6, we use a diamond membrane that hosts ensemble of NV centers placed on top of a YIG slab and experimentally explore the coupling between the NV centers and magnons. We employ a device geometry where surface spin waves are prominent and are expected to interact with NV centers efficiently. With the longitudinal relaxometry experimental measurements, we estimate the NV-magnon coupling strength in terms of the real part of the self-susceptibility of an NV center mediated by magnons. This scheme of experimentally characterizing the hybrid quantum system’s relevant parameters will be of great use in future exploration of the NV-magnon hybrid quantum architectures